1. Field of the Invention
The present invention relates to audio signal transmission and amplification systems, specifically to an improved configuration of an output channel in an audio signal amplification system which increases the power delivered to the speaker and reduces audible noise.
2. Description of Relevant Art
In audio signal transmission and amplification systems it is generally desirable to increase the power of the audio signal that is delivered to the speaker while simultaneously increasing the signal to noise ratio. Noise and signal distortion can be introduced by a variety of sources including the system ground and environmental factors.
Sophisticated audio systems often use components that produce a so-called "balanced" signal to increase power while at the same time reducing or eliminating noise and distortion. In balanced systems, the original audio signal is used to create two signals which are substantially identical to the original audio signal but which are 180.degree. out-of-phase with each other. The phases may be used together to produce an amplified signal that is a function of the difference in the amplitude of the two phases. The amplified signal is thus effectively independent of the system ground and any noise or distortions introduced by the system ground: the quality of the amplified signal depends only on the relationship of the out-of-phase signals to each other. Therefore, noise is reduced or eliminated in a balanced system since only the signal components that are opposite each other in the two signals are fully amplified. Common-mode signals, which may be caused by environmental factors, appear equally and with the same polarity on both out-of-phase signals, and are eliminated completely in an ideal balanced system because such common-mode signals have no effect on the difference in amplitude between the out-of-phase signals. Furthermore, if noise or distortion appears on only one of the out-of-phase signals, the magnitude of the noise or distortion is reduced, as it is not fully reflected in the difference between the signals. Thus the use of balanced signals results in an increase in the signal-to-noise ratio.
Audio signal transmission systems have usually been "balanced" through the power amplifier, such that the preamplifier produces the balanced signals which are then amplified by the power amplifiers. In standard systems the connections between the power amplifiers and the speaker have been not been balanced. The positive and negative output terminals of the power amplifiers are simply connected to the positive and negative input terminals, respectively, of the speakers. Some high-end amplifiers have been balanced through the speakers by using connections internal to the amplifiers; however, these amplifiers are expensive, and are non-standard components.
One method that has been used in some systems to increase power at the speaker using standard components is known as "bridging". In bridging, same polarity output terminals of two power amplifiers are connected to each other, and the terminals of one such power amplifier is connected to the positive and negative input terminals of the speaker, which effectively results in the output signals of the two power amplifiers to be additively applied to the speaker. While this method does increase the power delivered to the speaker, it does not eliminate noise, and the input signal to the speakers is dependent upon system ground.